The naval mine has been a mainstay of modern warfare. The North Sea Mine Barrage, a large minefield laid by the U.S. Navy and Royal Navy between Scotland and Norway during World War I inhibited the movement of the German U-boat fleet. Mines released by U.S. Navy submarines and dropped by U.S. Army Air Forces B-29 bombers in the Western Pacific during World War II sank hundreds of Japanese warships, merchant ships, and smaller vessels. Enemy-laid mines also took a high toll of Allied ships in both world wars.

Thus, when President Harry S. Truman ordered the U.S Armed Forces into action in Korea at the end of June 1950, American naval leaders took steps to deal with the mine threat. In early July, Vice Admiral C. Turner Joy, Commander Naval Forces, Far East, ordered Mine Squadron 3 to clear enemy mines from the approaches to Pohang on Korea's east coast. The UN command was desperate to stop the southward advance of the North Korean People's Army and needed troops quickly deployed ashore at Pohang. Accordingly, Lieutenant Commander D'arcy V. Shouldice, in command of fleet minesweeper Pledge and motor minesweepers Kite,Chatterer, Redhead, Partridge, Osprey, and Mockingbird, carried out a sweep of the approaches to the port and confirmed that the Communists had not mined those waters. As a result, on July 18, 1950, the U.S. Army's 1st Cavalry Division disembarked and soon added its combat power to that of the units on the Pusan Perimeter. Minesweepers also ensured that there were no mines around the essential port of Pusan.

The UN command was fortunate that it did not have to deal with enemy minefields in the first critical months of the war, because the allied navies did not have adequate mine countermeasures resources. Owing to post-World War II demobilization and defense budget cuts, the wartime force of 500 mine warfare vessels, manned primarily by Naval Reserve sailors, had been reduced to a worldwide contingent of two destroyer minesweeper divisions, two fleet minesweeper divisions, and 21 smaller craft. During the postwar years the Navy devoted much more of its attention and resources to the development of new aircraft carriers, jet aircraft, and shipboard surface-to-air missile systems than mine warfare ships and equipment.

The freedom from enemy sea mines did not last long. As allied forces carried out operations in preparation for the large-scale amphibious assault at Inchon, they discovered enemy minelaying activity. On September 4, U.S. destroyer McKean spotted mines in the water near Chinnampo and several days later two British combatants destroyed a number of floating mines in the same area. Then, on September 10, the crew of Republic of Korea Navy (ROKN) PC 703, a former U.S. Navy submarine chaser, caught an enemy boat dropping mines in the water in the approaches to Inchon. Commander Lee Hung So's ship dispatched the Communist minelaying vessel with one shot from her deck gun. Allied warships enroute to the landing sites at Inchon on September 13 and 14 also spied mines piled on the shore ready for laying or already in the water. As they continued their passage up Flying Fish Channel, destroyersMansfield, DeHaven, Lyman K. Swenson, and Henderson used their guns to eliminate the fearful mines. The North Koreans had begun their minelaying operation too late to stop General Douglas MacArthur's masterful amphibious landing at Inchon.

No sooner were UN ground troops safely ashore at Inchon, however, than Soviet-made mines began to take a toll of allied ships along the periphery of the Korean Peninsula. On September 26, off North Korea, U.S. destroyer Brush struck an enemy mine that killed 13 sailors, wounded 34 more, and put the ship out of action. Two days later YMS 509 of the ROKN sustained damage from a "floater" on the south coast. The next day Mansfield, spared at Inchon, hit a mine in North Korean waters that sent her to a shipyard in Japan. Then, in one day, October 1, Communist mines destroyed the wooden-hulled U.S. minesweeper Magpie, killing or injuring her entire crew of 33 men, and badly damaged ROKN YMS 504.

Sailors were killed and ships damaged in these separate incidents, but UN operations were not seriously disrupted. This could not be said of the planned allied landing at Wonsan, when an armada of 250 warships and transports, the latter carrying 50,000 marines and soldiers of the U.S. X Corps, waited idly offshore as minesweepers worked to clear an approach route through waters containing over 3,000 mines laid with the direct assistance of Soviet advisors.

On October 10, Captain Richard C. Spofford's Mine Squadron 3, warned by the helicopter crew from U.S. cruiser Worcester that mines were present in the waters off Wonsan, began their dangerous clearance mission. During that day and the next two, Spofford's nine vessels, helped by helicopter and PBM seaplane spotters, cleared a twelve-mile-long lane toward the landing site. The ships neutralized over 30 mines while the men of Underwater Demolition Team 3 marked another 50 mines for later destruction. In mid-morning of the 12th, however, minesweeper Pirate hit a mine and quickly sank, with six of her crewmen, and an hour later Pledgeand six of her sailors met the same fate. Enemy shellfire from shore addded to the danger and difficulty of the operation. By the 18th, two days before the planned landing, the minesweeping force had almost cleared all moored contact mines from the approach lane to the beach. That day, however, magnetic influence mines destroyed ROKN YMS 516 and half of her crew. Discovery of these new weapons stalled the operation. Finally, on October 25th the way was clear for the X Corps to deploy ashore at Wonsan. The operation, however, would be a non-combat "administrative landing," since Republic of Korea ground units had liberated Wonsan on October 11.

At one point Rear Admiral Allan Smith, in charge of the advance force at Wonsan, cabled the Navy's Washington headquarters that "we have lost control of the seas to a nation without a Navy, using pre-World War I weapons, laid by vessels that were utilized at the time of the birth of Christ." His words and the experience at Wonsan would engergize the Navy's mine warfare community.

Allied fortunes improved in the operation to clear mines from the approaches to Chinnampo on Korea's west coast. This port served Pyongyang, which was occupied by the fast-advancing U.S. Eighth Army on October 19th. The closer UN ground troops got to North Korea's border with the People's Republic of China in the late fall of 1950, the longer was the supply line from Inchon, so the ground units needed a port opened further to the north.

Consequently, even before the Wonsan operation was over, Admiral Joy ordered establishment of a new mine clearance force for Chinnampo. The ad hoc group included destroyer Forrest Royal, destroyer minesweepers Thompson and Carmick, small minesweepers Pelican, Swallow, and Gull (newly arrived from the United States), ROKN YMSs 502, 306, 513, and 503, LST Q-007, high speed transport Horace A. Bass with Underwater Demolition Team 1 embarked, dock landing ship Catamount carrying 14 minesweeping boats, 13 Japanese-manned minesweepers, and salvage ship Bolster. U.S. Navy PBM seaplanes and helicopters and Royal Navy Sunderland aircraft were used not only to spot mines but to destroy them with their machine guns.

The group began its work on October 28 when aircraft began searching for the more than 300 mines that North Koreans who had been involved in the minelaying operation claimed were in the water. On November 6, a Korean-manned tug, with U.S. Navy Commander Donald N. Clay on board, steamed from Chinnampo to the Yellow Sea to prove that the channel was free of mines. Then ROKN YMS 503 made the passage from the sea to the port. The docking of hospital ship Repose in Chinnampo on the 20th signalled the port's opening to seagoing support vessels. Not a man or a ship was lost carrying out the successful mine clearance operation at Chinnampo.

The stabilization of the fighting front around the 38th parallel from 1951 to 1953 ushered in a new era in mine warfare. Since the UN navies had driven Communist combatants from the sea early in the war, the enemy did what they could to deny the allies use of the waters off North Korea. The enemy made liberal use of sea mines along both coasts and covered their minefields with shore batteries. They deployed the mines from junks and sampans and released them in rivers that carried them to the sea. On occasion, Mother Nature aided the Communists, when fierce Asian typhoons tore mines loose from their moorings and spread them far and wide.

To threaten amphibious assaults, land guerrillas behind the lines, and open waters close offshore from which warships could bombard targets ashore, the UN mine clearance force often went in harm's way from 1951 to 1953. Nighttime sweep operations also became more frequent as did the seizure of enemy minelaying junks and sampans. These operations compelled the Communists to spread their forces and distracted them from concentrating on the ground war, but not without cost. From 1951 to 1953, mines sank U.S. minesweeper Partridge and tug Sarsi and ROKN ships YMS 306 and PC 704. More than twice that many UN mine clearance ships were damaged by mines or shore fire.

During the Korean War enemy mines caused 70 percent of all U.S. Navy casualties and sank the only four U.S. naval vessels lost in combat. The Korean War showed clearly that in the future the sea mine would be the weapon of choice for many of the U.S. Navy's adversaries and a fixture of late 20th century naval warfare.